The present invention relates to improvements in the rapid solidification of molten materials, including, but not limited to, metals, metalloids, and alloys.
For many applications, it is necessary that materials, including metals, metalloids, and alloys be provided in particulate form. Many systems have been devised for doing this. Among these are centrifugal atomizers which exist in various forms. In one form of centrifugal atomizers the material to be atomized is fed onto the surface of a rotating disc-like member which may be dished or flat. In one form of such systems a gas is used to cool the particles thrown off the rotating member by centrifugal forces. Representative of this type of system are U.S. Pat. Nos. 2,752,196, 4,053,264, and 4,078,873. Other systems rely on contact of molten droplets with a cooled surface.
In U.S. Pat. No. 4,347,199 granted Aug. 31, 1982 to Gentle and Speier, there is described a method and apparatus which provide a centrifugal atomizer making use of the heat of vaporization of liquid coolant and which thereby provides a system which offers rapid cooling of most components under equilibrium conditions at or near boiling point of the liquid coolant used. In U.S. Pat. No. 4,419,060, issued Dec. 6, 1983 to Liles and Speier a similar apparatus and method are taught, the primary difference being the location on the rotating disc at which the molten material is introduced. The solidified product produced by the apparati and methods of Gentle and Speier and Liles and Speier are, however, irregular and often flattened. This indicates that a major portion of the product produced by those methods is "splat cooled", i.e., the molten material cools and freezes while in contact with the rotating disc, builds up until its mass is such that it is thrown by centrifugal forces from the rotating device. This results in irregular particle shape and disproportionate particle size distribution.
Another splat cooling device is that described in U.S. Pat. No. 4,375,440, issued Mar. 1, 1983 to Thompson. In the apparatus described therein, molten metal is poured onto a spinning atomization disc means whereby liquid metal droplets leave the disc in a horizontal plane. An annular cooling gas jet flowing normal to the particle plane around the disc deflects the heavier liquid droplets to a conical splat plate which is fixed to rotate with said disc, whereby the droplets splat and cool, and are ejected by centrifugal force.
In U.S. Pat. No. 4,405,535, issued Sept. 20, 1983 to Raman et al., there is taught a method of preparing solid metal particles by contacting a molten stream of the material with a rapidly moving wall of a centrifugally disposed rotating liquid quench fluid, such as water or an oil, etc. In this manner, the stream of molten material is broken into molten globules or particles and rapidly quenched by the liquid. Raman et al. differs significantly from the instant invention by requiring the use of a liquid quenching fluid while the instant invention utilizes the more efficient atomized mist of coolant. In fact, Raman et al. is limited to liquid quenching fluids capable of being placed in the state of a rapidly moving centrifugally disposed rotating wall-like liquid mass and expressly teaches against atomization techniques.
Similarly, U.S. Pat. No. 2,439,772 issued on Apr. 13, 1948 to Gow, uses a revolving container containing a cooling or quenching liquid which from centrifugal force is forced into an annular vertical wall of revolving liquid into which are thrown globules of molten metal at a substantially normal horizontal path thereto to penetrate the liquid rather than glance off. See also U.S. Pat. No. 1,782,038, issued Nov. 18, 1930 to B. Haak, in which a melt of calcium nitrate salt was processed into globular bodies by centrifugation into a moving coolant bath of carbon tetrachloride.
In U.S. Pat. No. 4,078,873, issued Mar. 14, 1978 to Holiday et al., there is claimed an apparatus for producing metal particles by means of centrifugally throwing molten metal into an annular curtain of downwardly projecting cooling gas. In U.S. Pat. No. 4,377,375, issued Mar. 22, 1983 to Slaughter, there is taught a method and an apparatus for producing metal powders by rapid solidification of molten alloy. Slaughter atomizes molten alloy and centrifugally throws it into a stream of seed particles of solid material. The seed particles are impacted by the molten droplets causing the molten material to thinly deposit on the seed particles. Although this produces solidification, it has the disadvantage of producing larger particles due to the buildup on the seed particles.
T. Yamaguchi et al. (Appl. Phys Lett. 33(5), Sept. 1, 1978, p. 468-470) teaches preparation of amorphous powder by a water atomization technique in which molten alloy is introduced into the intersection of a pair of high velocity water jets.
The instant invention provides a method of solidification of molten material exhibiting greater efficiency of cooling, more uniformity of particle size and smaller resultant particles.